One of Yahoo!'s (NASDAQ:YHOO) choice landgrabs was to strike a deal to serve up display advertising on eBay (NASDAQ:EBAY) three years ago.

The deal has apparently been undone, according to JMP Securities analyst Sameet Sinha. And even though this has been a low-margin endeavor -- since eBay retains the lion's share of the money, as part of Yahoo!'s traffic-acquisition costs -- it could still cost $271 million to $305 million on Yahoo!'s top line.

The problem is that Yahoo! is bound to lose publishers. One of the problems with this summer's deal to let Microsoft (NASDAQ:MSFT) handle Yahoo!'s paid-search business is that third-party affiliates may see this as the perfect time to switch to market leader Google (NASDAQ:GOOG).

Google and eBay aren't strangers. Google's DoubleClick serves up the targeted text ads in eBay's ad-supported Kijiji.com classified website. If Sinha's claims -- as passed on by Barron's columnist Eric Savitz -- are true, this is a major opportunity for Google.

Even if running ads on third-party sites is a margin-nibbler, it still allows Yahoo! or Google to move some of its ad inventory. Advertisers also like to see prolific publishers on ad networks. Google doesn't need to sell itself too hard, though having eBay on its roster will have a magnetic effect.

Google has begun to put some muscle behind its display-advertising platform, so eBay will come in handy. Online players that specialize in display advertising will wince. Yahoo!, Time Warner's (NYSE:TWX) AOL, and ValueClick (NASDAQ:VCLK) probably figured that Google would focus on its more lucrative paid-search business.

Right? Wrong! Apparently, you can feed Google all you want and it will still have a voracious appetite. I wonder what it will eat next.

Was Yahoo!'s paid-search deal with Microsoft's Bing a mistake? Let us know your thoughts in the comment box below.

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Longtime Fool contributor Rick Munarriz is a satisfied eBay user, with 177 positive feedbacks to show for it. He owns no shares in any of the companies in this story and is also a member of the Rule Breakers analytical team, seeking out the next great growth stock early in its defiance. The Fool has a disclosure policy.